The present invention relates to the technical field of the treatment of exhaust gases, in particular the treatment of nitrogen oxide-containing exhaust gases.
The present invention relates in particular to a method for the treatment of nitrogen oxide-containing exhaust gases from technical processes, such as flue gases, for the purposes of removing and/or separating off the nitrogen oxides and/or for the purposes of reducing the nitrogen oxide content with chemical reduction of the nitrogen oxides. In particular, the present invention relates to a method for the denitrification of exhaust gases from large technical installations, such as for example power plants, in particular combined heat and power plants, or waste incineration installations.
Furthermore, the present invention relates to an apparatus for the treatment of nitrogen oxide-containing exhaust gases from technical processes, such as flue gases, for the purposes of removing and/or separating off the nitrogen oxides or else for the purposes of reducing the nitrogen oxide content by means of chemical reduction of the nitrogen oxides.
Furthermore, the present invention relates to the use of an apparatus for removing and/or separating off nitrogen oxides from nitrogen oxide-containing exhaust gases from technical processes, in particular flue gases.
In the case of combustion reactions in the presence of air, metastable, generally poisonous and reactive oxides of nitrogen, so-called nitrogen oxides, are formed. The formation of nitrogen oxides is intensified by the combustion or thermolysis and pyrolysis of organic and inorganic nitrogen-containing compounds, such as occur in large combustion installations such as combined heat and power plants or waste incineration installations.
Nitrogen oxides, in particular the compounds nitrogen monoxide and nitrogen dioxide, which are known under the designation “nitrous gases” and which are also referred to for short as NOx, are however not only poisonous and lead to irritation and damage to the respiratory organs, but also promote the formation of acid rain, as they react with moisture to form acids.
The release of nitrogen oxides is however also a problem for additional environmental protection reasons, as they firstly promote the formation of smog and hazardous ground-level ozone and secondly, as greenhouse gases, intensify global warming.
Owing to the disadvantageous effects of nitrogen oxides on health and on the environment, and not least owing to the associated economic damage, it has already long been attempted to minimize or prevent the release of nitrogen oxides from combustion processes. In passenger motor vehicles, this is achieved for example through the use of catalytic converters, which permit an almost complete removal of the nitrogen oxides from the exhaust gases.
To reduce the nitrogen oxide emissions from large technical installations, in particular from large industrial combustion installations, taking into consideration the respective legal situation and business considerations, various methods for denitrogenization or denitrification (DeNOx) have been developed which, on their own or in combination, are intended to yield an effective reduction or elimination of nitrogen oxides in exhaust gases, in particular flue gases.
Methods or measures for reducing the nitrogen oxide content of exhaust gases, in particular flue gases, can in this case be divided into primary and secondary measures:
In the case of the primary measures, the combustion process is controlled such that the nitrogen oxide content in the resulting exhaust gases is as low as possible; the formation of the nitrogen oxides is as far as possible suppressed, or nitrogen oxides that are formed are broken down as immediately as possible, but at the latest before exiting the combustion chamber. Primary measures include, for example, flue gas recirculation, wherein the flue gas is conducted again into the combustion zone, and air and fuel stages, wherein the combustion is controlled such that different combustion zones with different oxygen concentrations are realized. Furthermore, the formation of nitrogen oxides in flue gases can also be reduced through the addition of additives or by quenching, that is to say by the injection of water for the purpose of lowering the temperature during the combustion process.
By contrast to the primary measures, which are intended to prevent the formation of nitrogen oxides, the concentration of the nitrogen oxides in the exhaust gases, in particular flue gases is reduced through the use of the secondary measures. Secondary measures include for example separation methods, in which the nitrogen oxides are chemically bound or scrubbed out of the flue-gas flow. A disadvantage of the separation methods is however that large amounts of waste products are generated, such as for example process water, which are often contaminated with further flue gas constituents and must be disposed of in a costly manner.
Therefore, in modern large technical installations, as secondary measures, use is normally made of methods which are based on a reduction of the nitrogen oxides to form elementary nitrogen and which produce only small amounts of waste substances, wherein a distinction is generally made between catalytic and non-catalytic methods.
The selective catalytic reduction (SCR) of nitrogen oxides encompasses catalytic methods in which the nitrogen oxides are converted to form elementary nitrogen with the aid of metal catalysts. With SCR methods, it is generally possible to attain the best denitrification values, although the use of the catalyst makes the method considerably more expensive and less economically viable.
Furthermore, installations for carrying out the SCR method are extremely expensive not only in terms of purchase but also in terms of maintenance, as the sensitive catalytic converters must undergo maintenance, or be replaced, at short time intervals. Specifically in the case of large combustion installations in which the fuel composition can often be determined only with inadequate accuracy, such as for example waste incineration installations, there is therefore always the risk of poisoning of the catalytic converters by contaminants in the flue gas. This risk can be reduced only through the implementation of additional expensive measures.
By contrast, selective non-catalytic reduction (SNCR) is based on the thermolysis of nitrogen compounds, in particular ammonia or urea, which then react with the nitrogen oxides in a comproportionation reaction to form elementary nitrogen.
Selective non-catalytic reduction can be carried out at considerably lower cost than selective catalytic reduction: the costs for the purchasing and maintenance of SNCR installations amount to just 10 to 20% of the costs of corresponding SCR installations.
A problem of the SNCR method is however that the effectiveness thereof does not come close to matching the effectiveness of catalytic methods, such that, for example in the event of a further reduction of the legally permitted limit values for nitrogen oxides in exhaust gases, in particular flue gases, most SNCR installations would no longer be allowed to continue operating.
A further disadvantage of the methods based on the selective non-catalytic reduction of nitrogen oxides is that excess reducing agent must be used, which reducing agent does not react completely, such that the exhaust gas contains a certain and in some cases not insignificant amount of ammonia. Excess ammonia in the exhaust gas must either be separated off, or reduced in terms of content by process engineering measures, so as to enable the exhaust-gas flow to be released to the environment.
Furthermore, there are also methods which are based both on a catalytic effect and on the use of reducing agents, though in the case of these methods, too, the main disadvantages of the respective methods (high costs for the use of catalytic methods, and low effectiveness for the use of reducing agents) cannot be overcome.
Of late, novel SNCR installations and SNCR methods have been developed which are based on the combined use of multiple reducing agents and which exhibit effectiveness equivalent to that of catalytic methods, but such installations and methods however cannot provide optimum results at all times under all operating conditions.
For example, the temperatures required for SNCR methods, which lie in the range from 900 to 1050° C., are often disadvantageous because such high temperatures necessitate a treatment or denitrification of the flue gases before the flue gas enters the region of the heat exchangers. As a result, in particular in the case of combustion boilers being retrofitted with SNCR installations and during the operation of combustion boilers under full load, it is often the case that an injection of the reducing agent in a temperature range expedient for the SNCR method is not possible owing to the design of the boiler, or the temperatures that are expedient for the reduction are attained in the region of the heating surfaces or heat exchangers. In these cases, a major part of the flue gases, which may amount to up to 50% of the flue gas volume, often cannot be reached by the reducing agent, or the reducing agent must be introduced into the exhaust-gas flow in an unfavorable temperature range. Furthermore, with the use of urea as reducing agent, there is, in the region of the heat exchangers, the risk of deposition of ammonia or ammonium salts and thus of corrosion.
The introduction or injection of the reducing agent into the flue-gas flow in the optimum temperature range is however critical for the effectiveness of the nitrogen oxide reduction and thus for the efficiency of the denitrification.
With the injection of the reducing agent above 1100° C., the reducing agent is increasingly oxidized to form nitrogen oxides, whereby firstly the nitrogen oxide rate of separation decreases and, secondly, the consumption of reducing agent increases. By contrast, if injection is performed at excessively low temperatures, the reaction rate decreases, whereby so-called ammonia slippage occurs which results in the formation of ammonia or ammonium salts. This gives rise to secondary problems such as, for example, contamination of the fly ash with ammonia or ammonium salts, the amount of which is considerably increased, and the disposal of which is cumbersome and thus expensive. Furthermore, stringent legal limit values apply to the ammonia content of the purified exhaust gas, in order to as far as possible prevent damage to the environment.
Also, operationally induced temperature gradients in the boiler, that is to say large temperature differences and different flow speeds in a plane perpendicular to the flow direction of the exhaust gases, have the effect that the reducing agents are not distributed uniformly over the entire boiler cross section. It is thus always the case that reducing agents are injected into flue gas regions which lie outside the effective temperature window or range. This in turn results in inadequate nitrogen oxide reduction, high reducing agent consumption, and a high level of ammonia slippage.
To compensate temperature gradients and shift the temperature window for the reduction of the nitrogen oxides by means of the SNCR method into the region upstream of the heat exchangers even at full load of the boiler, methods have been developed in which water is injected into the flue-gas flow in order to cool the latter. Good results can be obtained in particular by means of a selective, that is to say spatially and temporally limited, addition of water into the flue-gas flow.
The injection of water for cooling the flue gases offers the great advantage inter alia that it is often possible to dispense with cumbersome and expensive boiler conversions if the flue gases are cooled to the preferred reaction temperature of the reducing agent before said flue gases enter the heat exchanger. This however has the major disadvantage that, depending on flue-gas temperatures and operating hours in which the boiler is operated in the upper load range, the boiler efficiency is impaired owing to water evaporation in the flue gas.
The efficiency of the heat exchangers and thus of the energy recovery, for example in combined heat and power plants, is consequently considerably reduced. For example, the loss arising from an injection of water as coolant in the context of the SNCR method is up to 3 MW in the case of a 225 MW combined heat and power plant alone, which illustrates the extent to which cooling of the exhaust gases reduces the efficiency of the power plant as a whole. For this reason, too, it is always sought to minimize the quantity of water introduced into the flue-gas flow during the implementation of the SNCR method.
An exacerbating factor in the denitrification of exhaust gases, in particular of flue gases, is furthermore that the exhaust-gas flow is not homogeneous, but rather has different temperatures, different gas speeds and different concentrations of exhaust gases at different locations and is subject to intense fluctuations over time. Owing to these chaotic and constantly changing conditions, incorrect dosing of the reducing agent into the exhaust-gas flow commonly occurs during the implementation of the denitrification method, in particular of the SNCR method, such that either the ammonia slippage is increased or the effectiveness of the method is reduced.
Furthermore, there has hitherto been no concept available for enabling the introduction of cooling water and reducing agent to be adapted promptly, preferably simultaneously, to the quickly changing conditions in the flue-gas flow.
It would therefore be desirable to have available an SNCR method which makes even greater allowance for the abovementioned points than the hitherto known methods from the prior art, and which makes it possible to considerably reduce the consumption of cooling water and reducing agent, wherein, at the same time, the ammonia slippage is further considerably reduced.